7.4 Temperature, Salinity and Currents in Jamaica Bay

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1 7.4 Temperature, Salinity and Currents in Jamaica Bay Arnold Gordon, Bruce Huber and Robert Houghton INTRODUCTION Jamaica Bay stratification is weakly indicative of a shallow, tidally active environment, with flood tide dominating slightly over ebb flow. Tidal excursions cause a non-stationary field, as water column properties sweep back-and-forth. Removing the spatial element with a temperature-salinity (T/S) relationship reveals the blending of the three inputs to Jamaica Bay water: coastal water (New York Bight), freshwater input (mostly treated sewage) and sea-air fluxes. We have two realizations of the T/S distribution: June and September 2000, defining the beginning and end of summer conditions, respectively METHODS Profiles of temperature and salinity with depth were collected at numerous sites within Jamaica Bay in June and September Sampling sites are summarized Table and in Figure The profiles were obtained with a Sea-Bird Electronics SBE19 Seacat CTD, sampling at a rate of 2 samples per second, yielding an effective vertical resolution of approximately 25 cm. Post-project calibration of the instrument is being carried out as of the writing of this report, so final estimates of instrument uncertainty are not yet available. However, previous experience indicates uncertainties (1 std dev) for temperature, salinity and pressure of 0.01 C; 0.02 psu and 0.3 decibars. The CTD stations were carried out from the NPS vessel Herbert Johnson in early June, and thenceforth from the US Park Police Marine Unit vessel. An effort was made to reoccupy several selected sites to enable comparison of tidal and seasonal influences. An acoustic current meter with integrated CTD was deployed 7 June - 19 July 2000 near the bridge in Rockaway Inlet, to monitor tidal fluctuations and to attempt to derive a mean flux estimate at the inlet. The instrument used was a Falmouth Scientific 2D-ACM/CTD, mounted in a weighted PVC frame which was deployed on the inlet bottom. The frame was recovered by Park Service Police divers. The CTD components failed prematurely due to flooding through the pressure port. The mooring frame was evidently knocked over on or about 4 July, presumably by recreational fishing. 46

2 Table Jamaica Bay CTD Profiles - LDEO Canarsie Date Yearday Number of High Tide Low Tide Profiles 06/01/ :59 13:49 06/02/ :50 14:41 06/07/ :35 19:13 06/08/ :33 20:20 06/09/ :29 21:28 07/19/ :57 17:05 09/11/ :30 13:15 09/12/ :10 13:59 09/13/ :47 14:42 09/14/ :22 15:24 09/15/ :56 16: RESULTS September: The T/S ( Figure 7.4-1) depicts a three point mixing environment. For salinity less than 26.5 (mainly eastern and northern Jamaica Bay) the temperature resides in the C range, 5 to 6 C warmer than the June condition. This represents the equilibrium surface temperature for the regional sea-air forcing. For salinity greater than 26.5 (western and southern Jamaica Bay) the temperature decreases with increasing salinity, with the most saline water of slightly over 30, at Rockaway Inlet floor, representing the coastal water end-member. At 26.0, below a depth of 5 meters Grassy Bay is filled with cooler water, C. As this is 5 C above the June temperature at the bottom of Grassy Bay (17.5 C), the isolation is clearly less than three months. June: The T/S scatter depicts a similar pattern, but regional equilibrium to a fixed SST has not yet been reached. Cool, saline coastal water end-member is slightly less than 16 C and 30 salinity. The warm, fresher end-member is about 19 to 20 C and salinity, and is located within the Grassy Bay surface layer. Cool (<18 C and relatively freshwater (<25) is found throughout. The origin of these waters is not yet clear. An advection/diffusion model as follows is envisioned: cool, saline coastal water flows into Jamaica Bay via Rockaway Inlet; freshwater is injected at various points via treated sewage, with Grassy Bay in NE Jamaica Bay representing the area most remote from the coastal end-member, most strongly influenced by freshwater input; strong vertical mixing couples the inflow and outflow throughout Jamaica Bay. Below 5 meters, Grassy Bay water is relatively isolated from the more active advective environment of the shallower layer. A simple salinity mixing recipe (assumes no net sea-air freshwater flux, not necessarily a good assumption for the rainy summer of 2000), finds that the coastal end-member is about 4 times that of the freshwater flux. If the volume of Jamaica Bay above 5 meters is 50 x 10 6 m 3 and the freshwater flux is 50 m 3 /s, then the bulk residence time of Jamaica Bay (not counting the portion of Grassy Bay below 5 m) is: 7 days. 47

3 Jamaica Bay - June & Sep 2000 Dye Studies Temperature Salinity Figure Temperature-Salinity distribution from the June and September, 2000 CTD stations. T/S points are color coded by region within the Bay. June points are displayed as '+' marks and September points as 'o'. Sample sites are identified in the inset map. 48

4 Figure Data from Rockaway Inlet current meter/ctd mooring June-July Mooring: The current meter/ctd mooring was deployed in Rockaway Inlet on 7 June and recovered 19 July. The moored CTD data ( Figure 7.4-2) clearly show a seasonal trend toward warmer, more saline conditions. Salinity peaks at high tide. The east-west speeds vary from 35 cm/s to the west to 55 cm/s to the east, with effective a zero mean (3.8 cm/s towards the west). When the zonal flow component was towards the east, the meridional speed was near zero; when the zonal flow was towards the west the meridional flow was towards the north. This is likely due to the siting of the mooring within 0.5 m of the bottom, and also may be due to its placement on the south side of Rockaway Inlet and may not represent the mean coupling of coastal and Jamaica Bay waters. Tidal effects: It is clear from the CTD sampling and moored data that a comprehensive study of Jamaica Bay must include a thorough assessment of tidal effects on stratification and circulation. While this study was not designed to undertake such an assessment, the data show qualitatively the impact of tidal flow on the stratification and points in and near the inlet ( Figure 7.4-3). Future studies will require careful examination of tidal influences on data collected to avoid tidal aliasing of time-series. 49

5 N W 2000/09/15 13: N W 2000/09/15 17:57 Temperature Pressure 10 te sa st High Tide (25901) Ebb Tide (25929) Salinity Density (st) Figure CTD data at a single site collected at different tide stages. Note the significant impact on stratification. 50